The calcium fluoride mineral used for the production of anhydrous hydrofluoric acid (HF) has several levels of contaminants including arsenic, which is mainly in the form of arsenic oxide (As2O3) . This compound is converted into arsenic trifluoride (AsF3) during HF production, this species remaining throughout the process and exiting together with the final product, the concentration of which depends on its entry concentration.
The presence of AsF3 in hydrofluoric acid is undesirable for those processes in which arsenic can be oxidized and change into the arsenic pentafluoride (AsF5) form, which is a very aggressive compound that can damage the integrity of the equipment exposed. For example, in the coolant manufacturing process, AsF3 is converted to AsF5, which poisons the catalyst and attacks metals. Therefore, it is desirable to minimize the presence of arsenic in hydrofluoric acid.
There are large deposits of fluorite with high arsenic content. To use the material these sources different processes have been used for the purification of HF, among which are the following:
Oxidation. For removing AsF3 from HF processes have been developed whose principles are oxidation reactions such as those listed in patent U.S. Pat. No. 3,166,379 (Bradley), which uses iodine, bromine or chlorine and an oxidizer such as potassium permanganate, or patent U.S. Pat. No. 3,689,370 (Osaka) using an oxidant such as permanganate and an iron salt in addition to impurities formed by the oxidant. Furthermore, patent CN 1,931,709 (Yin Fuhua) also uses potassium permanganate plus distillation, absorption, and filtration to remove impurities from HF. There are also patents U.S. Pat. No. 4,032,621 (Meadows) using an oxidizing agent and a reducing agent (hydrogen peroxide) and U.S. Pat. No. 4,083,941 (Jayawant) using hydrogen peroxide and methanol or sulfuric acid. Patent RU 2,246,444 (Varfolomeev) also uses potassium permanganate and potassium bifluoride and is subsequently distilled to obtain anhydrous HF of acceptable quality. The use of elemental fluorine and chlorine as an oxidizing agent is also described in patents U.S. Pat. No. 4,491,570 (G. Wheaton) and U.S. Pat. No. 4,668,497 (Miki Nobuhiro) respectively. Another patent of Chinese origin, CN 101,597,032 (Dingjun) uses hydrochloric acid, potassium permanganate and sodium hydroxide to oxidize arsenic and through several distillation steps obtains electronic grade hydrofluoric acid. There is also patent U.S. Pat. No. 5,047,226 (R. Schon) using potassium permanganate and potassium bifluoride is added as ion donor.
Hydrogen peroxide is added in other methods of oxidation as an oxidizing agent and in addition, catalysts such as molybdenum, vanadium, phosphorus, or similar compounds are added, such as in patent U.S. Pat. No. 4,756,899 (Jenczewski et al).
There are two patents in Mexico with the same principle as the above, which are: MX 165,894 (Morgan, B. Barry) and MX 167,322 (B. Barry, C. Redmon). Patent U.S. Pat. No. 4,929,435 (Kunkel-Redmon) further uses hydrogen peroxide, ammonium molybdate and sodium tripolyphosphate catalyst in a reactor with external cooling system, is subsequently heated and then distilled generating low-As HF.
The use of potassium permanganate for oxidation is still recurrent in patents, but they also perform a distillation with an ion purification column as mentioned in patent U.S. Pat. No. 6,063,356 (Hoffman) where a distillation is performed and potassium permanganate, ammonium persulfate and a potassium fluoride salt are added with an HF ionic purification.
In addition to the above oxidation methods, in patent U.S. Pat. No. 4,668,497 (Miki) oxidation of HF impurities such as arsenic is carried out using elemental fluorine in the form of gas mixed with other inert carrier compounds and subsequently separation is performed by distillation. This same method was developed in patents: JP 2005-281048 (Z. Hiroto) and U.S. Pat. No. 5,362,469 (Seseke-Koyro).
As to patent MX 171,054 (Ziegenhain) the oxidant used to convert trivalent arsenic to pentavalent arsenic is hexavalent chromium oxide and oxygen, subsequently distilled to recover pure HF.
Unlike the above, patent U.S. Pat. No. 6,346,227 (Lailach) first uses a distillation column to remove low boiling point compounds, and then adds elemental fluorine to oxidize arsenic and subsequently another distillation at ambient pressure. Something similar to patent EP 1,195,354 (Reinhold, Lailach, Peters) wherein trivalent arsenic is oxidized by amines and/or activated ammonium, and a double distillation with carbon steel columns and pressures of 0.9-1 bar-a.
Patent MX 162,803 (Garder) refers to an oxidation by adding elemental chlorine and anhydrous hydrogen chloride. Also patent CN 101,597,032 (Dingjun XIAO) speaks of the removal of impurities (including As) using anhydrous hydrochloric acid. Additionally, patent 171,259 MX (Gumprecht) uses sulfur to oxidize arsenic and in patent MX 172,980 (Gumprecht) uses iodine to achieve the same end.
Permeation. Patent U.S. Pat. No. 4,424,067 uses permeation membranes operating at high pressures. Electrolytic oxidation. Patents U.S. Pat. No. 5,100,639 (Freire) and U.S. Pat. No. 5,108,559 (Motz) refer to electrolysis processes for purifying hydrofluoric acid.
Similarly, patents ES 2,077,842 (Craig), VE 1991-000249 (Genieve), CN 1054574 (Freire) uses electrolysis for removal of arsenic in hydrofluoric acid.
Furthermore, there is Spanish patent ES 2,090,599 (Eileen) wherein an electrolytic oxidation is performed and subsequently the solution is distilled to obtain low arsenic HF. In addition to these, there are patents ES 2,077,842 (Barker), U.S. Pat. No. 5,108,559/U.S. Pat. No. 5,100,639 (Freire) that use the same principle for oxidation.
Evaporation. Patent U.S. Pat. No. 5,785,820 (Hoffman) uses a ionic purifier to remove impurities from HF; likewise, patent U.S. Pat. No. 5,722,442 (Hoffman) uses ammonia for the same purpose.
Filtration. Patent U.S. Pat. No. 4,952,386 (Davison) discloses a filtration through a cation exchange material and anion exchange material to remove arsenic ions.
Distillation. Oxidation processes form further compounds that are usually commercial impurities for hydrofluoric acid, so that an additional distillation process is required for separation thereof.
In addition, oxidants may form rapid decomposition unstable compounds that can cause problems in the safety of operation. As an alternative to this processes are those described in patents U.S. Pat. No. 3,663,382 (Garris) which describes a distillation process operating at low pressure (less than 25 psia) and patent U.S. Pat. No. 3,687,622 which discloses a distillation process operating at high pressures (above 165 psia). These processes have the disadvantage of operational cost because a lot of energy is required for both evaporation and condensation thereof. Furthermore, the process described in said patents starts from a concentration of 72 ppm at 100% reflux.
Additionally, we also have patent JP 2000-239002 (Guenter) in which HF is distilled at low pressures (14.2 psia) and an acid is obtained with a 60 to 90% concentration by weight. There is also patent US 2002-0001553 (Bulan Andreas) patent in which only the bottoms of the distillates are treated by evaporation and then neutralized with calcium salts.
In patent MX 284,844 (A. Juarez), apart from distillation under special conditions, use is disclosed of a second column in order to concentrate the purge and generate an arsenic byproduct with a +3 valence, (As+3) which is neutralized with calcium hydroxide (Ca(OH)2) to form CaF2 and As2O3, both products in solid form which are difficult to separate by conventional means.
The present invention proposes a high energy-efficient system for purification of HF, yielding an AsF3 concentrate product of up to 98% by weight. Also, it provides a solution for the safe handling of this product while maintaining the potential to turn it into arsenic commercial products such as arsenic trioxide (As2O3) and lithium hexafluoroarsenate (LiAsF6), which are high commercial value compounds.